1. Field of the Invention
This invention relates to an apparatus for an electronic measurement of the speed of internal combustion engines, comprising a computer, which serves to compute the speed and is connected to receiving means for detecting the passage of reference marks and of measuring scale graduations on a member which revolves with the crankshaft, and to a timer. This invention relates also to a method of operating such apparatus in monitoring an internal combustion engine.
2. Description of the Prior Art
Apparatuses of that kind are mainly used in combination with electronic control systems for metering the fuel, for determining the ignition timing and for performing other control measures for controlling the engine. The most important information for such control systems is the information on the instantaneous speed of the motor. In that connection the term "speed" may also be used to describe the angular or circumferential velocity during each revolution of the engine. Particularly in internal combustion engines having small flywheels, particularly in engines having a small volume and a small number of cylinders, that circumferential velocity varies relatively strongly as a result of the expansion of the combustion gases after the ignition and the compaction of the combustion gases before the ignition and the largest fluctuations occur when the speed is low and the load on the engine is high. Speed fluctuations over a plurality of revolutions may occur when the combustion is irregular or when the ignition and the fuel injection have intentionally been omitted during single revolutions. By means of the known apparatuses of the kind described first hereinbefore said fluctuations cannot be determined with sufficient accuracy so that the desired real-time control is adversely affected.
In known apparatuses of the kind described first hereinbefore the time of a revolution or the circumferential velocity of the crankshaft is detected for the speed measurement. In the first variant, a counter counting at a constant rate is basically provided and is controlled or read under the control of the reference marks provided on a rotating member so that the counter detects the repetition time of the reference marks. For the determination of the circumferential velocity, graduations which constitute a measuring scale are provided on the periphery of a member which rotates in unison with the crankshaft, e.g., on a very simple timing disk, and a timer is used to determine a gating time during which the graduations of the measuring scale moving past the receiver are counted. That count is proportional to the average circumferential velocity or angular velocity of the crankshaft but will not reflect the above-mentioned speed fluctuations over a plurality of revolutions so that such fluctuations cannot be evaluated.
Whereas it is known from DE-A-34 21 640 to store the measured values obtained during each revolution and to compare them with the measured values obtained during the next following revolution so that speed changes can thus be detected, the speed change is detected in that case only after it has taken place so that a control action cannot be taken in time.
From GB-B-2,198,241 it is known that inaccurancies in the pitch of graduations of a control disk or teeth of a control gear can be compensated in that correcting tables for the several graduations or teeth are stored in a computer, by which each measured-value signal generated during the passage of a mark or tooth is modified by a corresponding correcting signal so that the measurements will be taken in dependence on the corrected signals and will correspond to the measurements which would be obtained if the control disk or control gear had exactly the correct pitch. When the disks or gears are manufactured in series it will be assumed that the correcting values will also be the same so that the computer can be provided with fixedly programmed correcting tables.